ORCID Profile
0000-0002-4520-6147
Current Organisations
Tokyo Institute of Technology
,
University of Tokyo
,
Helmholtz-Zentrum Dresden-Rossendorf
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Publisher: American Chemical Society (ACS)
Date: 02-07-2009
DOI: 10.1021/IC9004467
Abstract: In order to elucidate the uranium solution chemistry at the high HNO(3) concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U(IV) and U(VI) are studied in aqueous HNO(3) solutions, as well as in HClO(4) solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO(4), U(IV) exists as a spherical cation of U(4+), which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U(IV)O(2) x nH(2)O, at a lower acidic concentration of 0.1 M HClO(4). U(VI) exists as a transdioxo uranyl cation, UO(2)(2+), and forms a 5-fold pure hydrate complex of [U(VI)O(2)(H(2)O)(5)](2+) in 1.0 M HClO(4). With increasing HNO(3) concentration, the water molecules of the U(IV) and U(VI) hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO(3)(-)), forming dominant species of [U(IV)(H(2)O)(x)(NO(3))(5)](-) in 9.0 M HNO(3) and [U(VI)O(2)(NO(3))(3)](-) in 14.5 M HNO(3), respectively. The present multitechnique approach also suggests the formation of two intermediate U(VI) species, a 5-fold mononitrato complex ([U(VI)O(2)(H(2)O)(3)(eta(2)-NO(3))](+)) and a 6-fold dinitrato complex ([U(VI)O(2)(H(2)O)(2)(eta(2)-NO(3))(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO(3) concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U(VI) complexes is less probable in the present HNO(3) system.
Publisher: American Chemical Society (ACS)
Date: 15-06-2009
DOI: 10.1021/IC9003005
Abstract: The coordination and redox behavior of Np(IV), Np(V), and Np(VI) sulfate in aqueous solution were investigated by Np L(3)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances R(Np-S) and coordination numbers N(S) obtained by EXAFS spectroscopy. Np(VI) is coordinated by sulfate in the bidentate (R(Np-S) = 3.12 +/- 0.02 A) and monodentate (R(Np-S) = 3.61 +/- 0.02 A) modes at a low sulfate concentration of [SO(4)(2-)]/[NpO(2)(2+)] = 1. At higher [SO(4)(2-)]/[NpO(2)(2+)] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) with 2.0 M SO(4)(2-) and at pH 1.1. Np(V) is coordinated by sulfate in the bidentate (R(Np-S) = 3.16 +/- 0.02 A) and monodentate (R(Np-S) = 3.67 +/- 0.02 A) modes. However, sulfate coordination is less pronounced and does not exceed one SO(4)(2-) per Np(V) with 2.0 M SO(4)(2-). The redox reaction between the Np(VI)/Np(V) couple can be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R(Np-S) = 3.06 +/- 0.02 A) and monodentate (R(Np-S) = 3.78 +/- 0.02 A) coordination modes. The sulfate coordination increases with an increasing [SO(4)(2-)]/[Np(4+)] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th(IV)-U(IV)-Np(IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons.
Publisher: American Chemical Society (ACS)
Date: 23-01-2008
DOI: 10.1021/AC7021579
Abstract: A multitechnique approach using extended X-ray absorption fine structure (EXAFS) spectroscopy based on iterative transformation factor analysis (ITFA), UV-visible absorption spectroscopy, and density functional theory (DFT) calculations has been performed in order to investigate the speciation of uranium(VI) nitrate species in acetonitrile and to identify the complex structure of in idual species in the system. UV-visible spectral titration suggests that there are four different species in the system, that is, pure solvated species, mono-, di-, and trinitrate species. The pure EXAFS spectra of these in idual species are extracted by ITFA from the measured spectral mixtures on the basis of the speciation distribution profile calculated from the UV-visible data. Data analysis of the extracted EXAFS spectra, with the help of DFT calculations, reveals the most probable complex structures of the in idual species. The pure solvated species corresponds to a uranyl hydrate complex with an equatorial coordination number (CNeq) of 5, [UO2(H2O)5]2+. Nitrate ions tend to coordinate to the uranyl(VI) ion in a bidentate fashion rather than a unidentate one in acetonitrile for all the nitrate species. The mononitrate species forms the complex of [UO2(H2O)3NO3]+ with a CNeq value of 5, while the di- and trinitrate species have a CNeq value of 6, corresponding to [UO2(H2O)2(NO3)2]0 (D2h) and [UO2(NO3)3]- (D3h), respectively.
Publisher: Wiley
Date: 20-10-2023
Publisher: American Chemical Society (ACS)
Date: 28-05-2009
DOI: 10.1021/IC900917V
Publisher: American Chemical Society (ACS)
Date: 10-11-2007
DOI: 10.1021/IC701607E
Abstract: We studied the structure and stoichiometry of aqueous uranylVI hydroxo dimers and trimers by spectroscopic (EXAFS, FTIR, UV-vis) and quantum chemical (DFT) methods. FTIR and UV-vis spectroscopy were used for the speciation of uranyl complexes in aqueous solution. DFT calculations show that (UO2)2(OH)22+ has two bridging hydroxo groups with a U-U distance of 3.875 A. This result is in good agreement with EXAFS, where a U-U distance of 3.88 A was found. For the hydroxo trimer complex, DFT calculations show that the species (UO2)3(O)(OH)3+ with oxo bridging in the center is energetically favored in comparison to its stoichiometric equivalent (UO2)3(OH)5+. This is again in line with the EXAFS result, where a shorter U-U distance of 3.81-3.82 A and evidence for oxo bridging in the center were found. Several stable intermediates which lie several tens of kJ/mol above that of (UO2)3(O)(OH)3+ were identified, and their structures, energies, and intramolecular proton-transfer reaction are discussed.
Publisher: Royal Society of Chemistry (RSC)
Date: 2012
DOI: 10.1039/C2DT12406H
Abstract: Primary aquo species of tetravalent cerium (Ce(IV)) in perchloric acid has been identified as a single oxo-bridging dinuclear complex, not a mononuclear one, by extended X-ray absorption fine structure (EXAFS) spectroscopy combined with density functional theory (DFT) calculations.
Publisher: Walter de Gruyter GmbH
Date: 11-2008
Abstract: The coordination of U(VI) sulfate complexes has been investigated by uranium L III -edge EXAFS and HEXS measurements with the aim to distinguish monodentate and bidentate coordinated sulfate in aqueous solution. UV-vis absorption spectroscopy has been used to differentiate the species and to determine the species distribution as a function of the [SO 4 2− ]/[UO 2 2+ ] ratio. A monodentate coordination prevails in solutions with [SO 4 2− ]/[UO 2 2+ ] ratio of 1, where UO 2 SO 4 is the dominant species. Besides the dominating monodentate sulfate a small amount of bidentate sulfate could be observed, indicating that two isomers may exist for UO 2 SO 4 . With increasing [SO 4 2− ]/[UO 2 2+ ] ratio the UO 2 (SO 4 ) 2 2− species becomes the main species. The uranium atom of this species is coordinated by two bidentate sulfate groups.
Publisher: American Chemical Society (ACS)
Date: 07-04-2007
DOI: 10.1021/IC070051Y
Abstract: Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) s les and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)3]n- [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.
Publisher: American Chemical Society (ACS)
Date: 12-11-2009
DOI: 10.1021/IC901838R
Abstract: The electrochemical behavior and complex structure of Np carbonato complexes, which are of major concern for the geological disposal of radioactive wastes, have been investigated in aqueous Na(2)CO(3) and Na(2)CO(3)/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na(2)CO(3) with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO(2)(CO(3))(3)](n-) (n = 5 for Np(V), and 4 for Np(VI)). Hence, the electrochemical reaction of the Np(V/VI) redox couple merely results in the shortening/lengthening of bond distances mainly because of the change of the cationic charge of Np, without any structural rearrangement. This explains the observed reversible-like feature on their cyclic voltammograms. In contrast, the electrochemical oxidation of Np(V) in a highly basic carbonate solution of 2.0 M Na(2)CO(3)/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [Np(VII)O(4)(OH)(2)](3-), indicating that the oxidation reaction from Np(V) to Np(VII) in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO(3)(2-)) to hydroxide ions (OH(-)).
No related grants have been discovered for Satoru Tsushima.